Next generation multimedia wireless communication system on which studies are actively undertaken recently is required to be capable of processing and transmitting a variety of information, such as image data, packet data and the like, more than providing early stage voice-oriented services.
One of the most fundamental problems to be solved to accomplish high speed communication is how efficiently and reliably data can be transmitted through a channel. In channel environment of a wireless communication system, unlike that of a wired communication system, loss of information is inevitable due to errors occurred by a variety of factors such as multi-path interferences, shadowing, electromagnetic wave attenuation, time-varying noises, interferences and fading, or the like. Such loss of information practically generates severe distortions in a transmission signal and acts as a factor that degrades overall performance of the system. It is general to enhance reliability of systems using a variety of error-control techniques depending on characteristics of a channel in order to reduce loss of information, and one of the error-control techniques is error-correcting code.
A codeword is a bit stream that is generated by performing a specific process (e.g. channel coding) on information bits comprising zeros and ones in order to enhance detection performance when the information bits are transmitted. In block channel coding such a Reed-Muller code, a codeword is generated through a matrix that is called as a generating matrix. In trellis channel coding such a convolutional code or a turbo code, a codeword is generated through a polynomial expression such as a generating polynomial.
A codeword used for channel coding should have a bit stream comprising more than a certain number of bits to guarantee its performance. Performance of detecting the codeword is depending on performance of channel estimation. If the length of the codeword is not enough long, non-coherent detection is widely used. In the non-coherent detection, the codeword is detected without performing channel estimation so that performance of channel estimation does not affect performance of detecting the codeword. On the contrary, the codeword is detected using channel estimation in the coherent detection.
In the non-coherent detection, it is necessary to use an orthogonal code that has superior characteristics of auto-correlation and cross-correlation in order to enhance detection performance of the codeword.
Various control signals are exchanged between a transmitter and a receiver in order to transfer traffic data in a wireless communication system. Control signals includes a channel quality indicator (CQI), an acknowledgement/negative-acknowledgement (ACK/NACK), a multiple input multiple output (MIMO) codebook index or the like. Since the control signals is used to determine modulation and coding scheme of the traffic data, the control signals should be successfully detected before the traffic data are transferred from the transmitter to the receiver.
In coherent detection, a control signal is detected after a channel is estimated and compensated. The control signal is converted into a codeword by performing channel coding. As the length of the codeword is the longer, the more excellent detection performance can be achieved. However, when the length of the codeword is not enough long, the detection performance can be significantly degraded. The detection performance is very sensitive to performance of channel estimation.
In non-coherent detection, a control signal is converted into a codeword by mapping to an orthogonal code. The codeword of the control signal is detected using a correlator instead a channel estimator and the codeword is demapped to the control signal. Although the length of a codeword in non-coherent detection is shorter than that of a codeword in non-coherent detection, detection performance in non-coherent detection can be relatively highly achieved. Furthermore, since the detection performance is not affected by performance of channel estimation, complexity due to the channel estimation can be greatly lowered.
The codeword that uses the orthogonal code has lack of extensibility since the length of the codeword is exponentially increases in proportion to the number of bits to be transmitted and complexity of the correlator increases as the length of the codeword increases.
In order to improve detection performance, a method for designing a codeword that is insensitive to variations of a channel, less complex and easily extensible is needed.